1. Field of the Invention
The present invention relates to a developing apparatus which is arranged to oppose a sheet-like recording medium having an electrostatic latent image formed thereon so as to cause a developing solution in which charged fine color particles (toner particles) are dispersed to act on a surface of the recording medium on which the electrostatic latent image is formed.
More particularly, the present invention relates to a roller type liquid developing apparatus of a negative pressure suction scheme, in which a developing solution is circulated/supplied to a developing head by using a negative pressure suction means, and an electrostatic latent image formed on a recording medium is developed by rotating a developing roller which is designed to carry the developing solution on its surface and to oppose the recording medium through a very small gap.
2. Description of the Related Art
For example, U.S. Pat. No. 4454833 discloses a developing apparatus wherein a rotatably supported developing roller is arranged to oppose a recording medium having an electrostatic latent image formed thereon through a very small gap, and the roller is rotated while at least a part of the roller is dipped in a developing solution so as to cause a developing solution layer carried on the developing roller surface to act on the electrostatic latent image, thus performing a developing operation.
In addition, for example, Published Examined Japanese Utility Model Application No. 52-25153 discloses a liquid developing apparatus of a negative pressure suction scheme wherein a developing solution is circulated/supplied to a developing head by using negative pressure suction of a suction pump.
In the roller type liquid developing apparatus disclosed in U.S. Pat. No. 4454833, a large amount of developing solution can be supplied while the developing roller is caused to approach a recording sheet surface so as to apply a strong electromagnetic effect onto the recording sheet surface. With this arrangement, a high developing efficiency can be obtained. However, a large amount of residual developing solution adheres to the surface of the recording sheet after the developing operation. For this reason, it is difficult to quickly and completely remove this residual solution by using only a developing solution removing roller. Consequently, in order to remove the developing solution adhering to the developing solution removing roller itself so as not to cause it to adhere to the recording sheet surface again, a considerable amount of recording sheets must be kept fed to sufficiently spread the residual developing solution on the recording sheet surface. Therefore, recording sheets are wastefully consumed in large quantities, posing an economical problem. In addition, since the outer surface of the developing roller as the upper surface of the developing head is externally exposed, a developing solution tends to be scattered outside. The surroundings inevitably tend to be smeared.
In U.S. Pat. No. 4454833, a blade for the developing roller is designed to support an elastic sheet, such as a polyurethane sheet or a polyester sheet, on its one end and to urge its other end against the surface of the developing roller. When the developing apparatus having such an arrangement is to be applied to a recording apparatus using a recording sheet having a width of about 1 meter, such as a color electrostatic plotter, a long developing head is required. For this reason, it is difficult to uniformly support the above-mentioned blade against the outer surface of the developing roller. Especially, it is not easy to ensure an accurate operation of the blade by causing it to follow the deflection of such a long developing roller.
If the developing roller is off-centered or bent due to such deflection, the developing gap varies to greatly influence the developing performance. In order to manufacture a long developing roller without deflection, its diameter and rigidity must be increased. Then, the developing head is increased in size and weight, and it is very difficult to process such a developing head. This presents an obstacle in realizing a small roller type liquid developing apparatus which is easy to manufacture.
In the roller type liquid developing apparatus disclosed in U.S. Pat. No. 4454833, the developing roller has a smooth surface without an uneven portion. For this reason, it is sometimes difficult to efficiently attach a developing solution to the surface of the developing roller. According to a proposed means for solving such a problem, a spiral groove or grooves parallel to the roller axis are formed in the surface of the developing roller. However, with the above-mentioned means, developing streaks are sometimes formed.
In the liquid developing apparatus of the negative pressure suction scheme disclosed in Published Examined
Japanese Utility Model Application No. 52-25153, since a negative pressure acts on a developing slit and the like during a developing operation, a developing solution on a sheet-like recording medium is sucked/removed. Such an effect serves to suppress the developing solution from scattering outside, thus reducing the possibility of smearing. However, if the developing operation is completed, and the suction pump is stopped, the above-mentioned effect is lost. Since some developing solution is attached to the surface of the sheet-like recording medium located at the developing slit at this time, if the medium is moved in this state, the developing solution will be spread over the surface of the medium. As a result, the surface of the recording medium or those of the convey rollers are smeared with the solution.
In order to solve such a problem, another means, e.g., a special suction drying head, must be arranged to remove a developing solution attached to a recording surface in a stop state of the pump. Another problem is that an excessively large amount of developing solution is attached to a recording medium and carried away at the end of developing, although a very small amount of developing solution is carried away by the recording medium in the process of developing.